Chapter 3 Groundwater in the Urban Environment13).pdf · of groundwater are thus necessities for...

Chapter 3Groundwater in the Urban Environment

Peter Shanahan

Groundwater is a critical resource for many of the world’s cities. While a few cities(for example, New York) rely upon protected surface-water reservoirs for their sup-ply, many more depend on groundwater. Conservation, protection, and managementof groundwater are thus necessities for most cities. This chapter reviews the basicsof groundwater hydrology, supply, and water quality, and then goes on to examinegroundwater in the specific context of the urban environment.

3.1 Introduction to Groundwater

Groundwater is but one component of the hydrologic cycle that moves water throughthe environment. Precipitation (rain and snow) carries moisture from the atmosphereto the land surface. Other than in arid areas, the majority of precipitation runs offthe land surface, concentrating in streams and then rivers and eventually reaches theoceans. Evaporation from the oceans carries water to the atmosphere once again,restarting the cycle. The portion of precipitation that does not run off the land surfaceis available to infiltrate into the ground. Some of that water evaporates or is takenup by plants which transpire water to the atmosphere. This combined loss of waterto the atmosphere is known as evapotranspiration. The portion of the water thatremains in the ground becomes groundwater and is known as recharge.

Groundwater lies beneath all landscapes in the empty space that exists betweensoil particles or in cracks in bedrock. Often, the soil near the surface is unsaturated,meaning that the space between soil particles is not completely filled with water.This water slowly seeps downward under the influence of gravity. Eventually, itreaches the water table, which is the top of the zone in which the soil is saturated

P. Shanahan (B)Department of Civil and Environmental Engineering at Massachusetts Institute of Technology,Cambridge, MA, USAe-mail: [email protected]

L.A. Baker (ed.), The Water Environment of Cities,DOI 10.1007/978-0-387-84891-4 3, C© Springer Science+Business Media, LLC 2009

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30 P. Shanahan

Fig. 3.1 Illustration of confined and unconfined aquifers and associated travel paths (Winter et al.,1998)

with water (i.e., all space between soil particles is filled with water). Conditions inwhich there is a water table are called unconfined, to distinguish them from con-fined, or artesian, conditions. In confined conditions, groundwater is kept underpressure by a capping layer of rock or soil that is too tight to allow water to flowthrough it easily (a confining bed). Figure 3.1 illustrates the concepts of confinedand unconfined aquifers and illustrates that multiple aquifers may lie beneath theland surface.

Groundwater exists in the subsurface in geologic formations of different char-acter. Often, it is useful to distinguish between bedrock formations and unconsol-idated formations (also called overburden). Bedrock is the basement of rock thatlies at depth beneath the land surface. Some types of rock, such as sandstone, aremade up of cemented particles that have openings between them. Others, such asgranite, have essentially no openings except where there are cracks, known as frac-tures, in the otherwise solid rock. Either of these types of rock can store and transmitgroundwater. Although bedrock may be at the land surface it is usually deeper andcovered by unconsolidated soil and geologic deposits in which the particles are notcemented together. For example, the sand and gravel that forms the bed of a riverwould be such an unconsolidated formation. Flowing water, glaciers, blowing wind,and landslides on mountain slopes are among the processes that can leave uncon-solidated deposits. Landfilling, digging, and earthmoving by man can also createunconsolidated deposits. As with bedrock formations, unconsolidated deposits canstore and transmit groundwater.

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3.1.1 Groundwater Flow

Groundwater in the saturated zone flows from areas of high potential energy (knownas potential, potentiometric head, or simply head) to areas of low potential energy.On a water table, this would be the same as flowing downhill from high water-tableelevations to low elevations. Under confined conditions, the concept is similar butthe confining bed keeps the groundwater from forming an actual water surface withhighs and lows. Instead, pressure builds up under the confining bed, with higherpotential in some areas than in others. The potential can be mapped with highs andlows of pressure on what is known as a potentiometric surface map much as actualelevations can be mapped on a water-table map. The water-table map is in fact atype of potentiometric surface map. Figure 3.2 shows an example potentiometricsurface map for historical conditions in the confined Chalk aquifer beneath Londonin the UK.

Fig. 3.2 Map of the potentiometric surface in the confined Chalk aquifer beneath London (Hiscock,2005). Potentiometric surface elevations are given in meters above mean sea level

32 P. Shanahan

Groundwater moves from any area of high potential to areas of low potential,whether horizontally or vertically. Thus, if potential decreases with depth under-ground, the groundwater will tend to move downward. Groundwater will similarlymove horizontally to areas of low potential. Often, the water table is a muted reflec-tion of the land surface and groundwater typically flows horizontally from areas ofhigh ground to areas of low ground, eventually exiting the subsurface to flow intostreams and rivers. This natural pattern can be disrupted locally where wells with-draw large quantities of groundwater. Because of the relationship between head andgroundwater flow, a potentiometric surface map can be used to infer the directionof flow. For example, in Fig. 3.2, the numbered lines on the map are lines of equalpotential head with the number indicating the elevation in meters relative to meansea level. The map can be read much like a topographic map of the land surface: asindicated by the arrows on the map, water flows from the higher heads at the periph-ery of the city into the city center where substantial pumping from wells lowers thepotentiometric surface.

Groundwater movement is restrained by frictional resistance to flow in the tightspaces between soil and rock particles. The ability of a soil to allow water movementis called its hydraulic conductivity or sometimes permeability. For example, waterhas a much more difficult time moving through tight clay than it does through muchmore open gravel and coarse sands. Sand and gravel is much more permeable (has amuch higher hydraulic conductivity) than clay. Hydraulic conductivity is in units ofdistance over time (for example, centimeters per second) and represents the amountof groundwater that will flow through a unit area (for example, a 1 cm by 1 cm areathrough which the water would flow) under a unit potential gradient (for example, awater table with a slope of 1 cm vertical over 1 cm horizontal). A hydraulic conduc-tivity of 1 cm/sec does not mean groundwater moves at a speed of 1 cm/sec; in fact,the speed of groundwater movement is almost always many, many times less thanthe hydraulic conductivity because hydraulic gradients are typically very small. Wediscuss groundwater velocities further below.

The relationship between hydraulic conductivity and the rate at which groundwa-ter moves from areas of high potential to areas of low potential is given by Darcy’sLaw. The law was discovered by a French engineer, Henri Darcy, when he undertooklaboratory tests in 1856 while working on the water supply for the city of Dijon,France. This simple but elegant equation is the fundamental law of groundwaterhydrology and therefore we include it in this otherwise non-mathematical textbook.Darcy’s Law states that the flow of water in a porous medium is given by:

q = Q

A= −K

�h

�L

where,

q is known as the specific discharge, the amount of water flowing per unit timethrough a unit cross-sectional area of the porous medium, and is given in unitsof length per time;


Q is the discharge, the total amount of water flowing, given in units of volumeper time;

A is the total cross-sectional area through which the water is flowing, given inunits of length squared;

K is the hydraulic conductivity, in units of length per time; and,�h/�L is the hydraulic gradient, which is the change in potentiometric head (�h)

over distance (�L). Since �h and �L are both in units of length, �h/�L isdimensionless.

Darcy’s Law indicates that the amount of water that will flow through a porousmedium is a function of the energy that drives the flow (as represented by the poten-tial gradient, �h/�L) and the degree to which the medium allows movement ofwater (captured by hydraulic conductivity, K). Hydraulic conductivity varies widelyfor different kinds of geologic materials. Going back to the examples given above,a tight, compact clay resists the movement of water and therefore has a low valueof K—about 0.00009 m/day. In contrast, water readily moves through coarse sandand gravel which has a high value of K—about 300 m/day. Table 3.1 shows typicalhydraulic property values for a variety of geologic materials.

These last paragraphs have emphasized a key concept in groundwater—theimportance of hydraulic conductivity in dictating the rate of groundwater flow. Inmost of the subsurface there is present a variety of different geologic materials orsoil types, often with widely varying hydraulic conductivity. Given two pathwaysfor flow, groundwater will tend to follow the pathway of least resistance—the path-way of highest hydraulic conductivity. Thus, if there is a seam of sand and gravelin an otherwise clayey soil, almost all of the groundwater will flow in the sand andgravel rather than the clay. These zones of high groundwater hydraulic conductivityare also the logical places to install groundwater supply wells.

In many instances, we are interested in the speed at which groundwater flows.Particularly, if groundwater is contaminated by a chemical substance, we often wantto know how fast the contaminated water will travel to a point where it can affect

Table 3.1 Typical hydraulic properties of geologic materials (based on Mercer et al., 1982)

Hydraulic Groundwater GroundwaterMaterial conductivity (cm/sec) Porosity velocity∗(m/day) velocity (m/year)

Gravel 4 × 10–1 0.34 5 1900Coarse sand 5 × 10–2 0.39 0.6 200Fine sand 3 × 10–3 0.43 0.03 11Silt 3 × 10–5 0.46 3 × 10–4 0.1Clay 1 × 10–7 0.42 1 × 10–6 0.004Weathered granite 2 × 10–3 0.45 0.02 7Basalt 1 × 10–5 0.17 3 × 10–4 0.09Sandstone 5 × 10–4 0.34 0.006 2

∗Assuming hydraulic gradient of 0.005.

34 P. Shanahan

people. The speed of groundwater movement is affected by the fact that water andchemical contaminants can only flow within the spaces between the rock and soilparticles. The space between the rock and soil particles is known as the pore spaceand the fraction of the aquifer taken up by pore space is known as the porosity. Thespeed at which groundwater flows through these pores, v, is the specific dischargedivided by the porosity or v = q/n, where q is the specific discharge that is computedusing Darcy’s Law and n is the porosity. Porosity is a fraction between zero and one,but is typically around 0.3 to 0.4 for sand. It can be much less in fractured bedrock,where only a small part of the rock is open space. Table 3.1 includes typical porosityvalues for porous media.

It is instructive to consider the speed at which groundwater moves in the sub-surface. Typically, the water table is gently sloped, falling perhaps 5 m in 1000 m.Thus the hydraulic gradient �h/�L is 0.005. Assume the soil is a fine sand, with ahydraulic conductivity of K = 0.003 cm/sec = 3 m/day. A typical porosity for finesand is n = 0.43. With these values, we can use Darcy’s Law to compute the speed ofgroundwater flow as v = K/n (�h/�L) = 0.03 m/day, only about 11 m/year. Ground-water moves slowly! Typical groundwater speeds for a range of geologic materialsare shown in Table 3.1.

3.1.2 Groundwater Supply

Water under the ground represents a resource that can be tapped to provide watersupply for individual homes, small villages, or large towns or industries. Accordingto the United States Geological Survey, approximately one quarter of the fresh waterused in the United States in the year 2000 originated from groundwater (Hutsonet al., 2004). Groundwater is also an ecological resource in that groundwater typ-ically flows into streams, providing the baseflow that sustains aquatic ecosystemsthrough dry spells. Finally, groundwater serves as a structural element in the sub-surface, contributing to the geologic integrity of the land. Removal of groundwatercan destroy this integrity and lead to subsidence and creation of sinkholes.

Groundwater supplies come from aquifers. Water-bearing zones in the subsur-face are called aquifers if they are sufficiently permeable to transmit readily usefulquantities of water to wells, springs, or streams under ordinary hydrologic condi-tions. Aquifers thus have economic value—they can provide a water supply to ahome, a village, or a city. However, an aquifer that can provide adequate water for asingle residence might not be able to supply a municipality. Thus, it is common toconsider the yield of an aquifer, which is the long-term sustainable quantity of waterthat may be withdrawn from it. Aquifers with large areal extent and high hydraulicconductivity have high yield.

Groundwater may naturally discharge to the land surface at springs or intostreams, however the most common means to extract groundwater is via a well.A water well is simply a hole dug or drilled into the earth and used to obtain water.In a pumped well, an electric or mechanical pump mechanism is installed belowthe water surface in the well and used to withdraw water. The removal of watercauses a localized lowering of the water table which, due to Darcy’s Law, causes


an inward flow of groundwater from the surrounding aquifer (Fig. 3.2). Aroundthe well, the water-table surface forms a characteristic shape, known as a cone ofdepression, which features a progressive lowering of the water table and steepeningof the hydraulic gradient inward to the well.

When multiple wells are located near each other, their cones of depression canoverlap and cause a general lowering of the water table. Figure 3.2 shows a contourmap of the potentiometric surface of the confined Chalk aquifer beneath London in1994. Prior to pumping, potentiometric elevations in the aquifer were above sea leveland wells in the Chalk aquifer flowed under artesian pressure. However, pumpingover time lowered the potential in the aquifer and by 1994 potential head was atleast 10 m below sea level through much of the city and over 40 m below mean sealevel in the city center.

Just as removing groundwater from wells can lower potential in the aquifer,adding water into wells can raise the potential. There is now an extensive aquiferrecharge system around London in which excess surface water is injected into theaquifer. Such artificial recharge schemes can be particularly useful in areas withlarge seasonal variations. Excess surface water can be recharged to aquifers duringwet seasons and stored so it is available for withdrawal during the dry season. Sim-ilarly, treated wastewater is now being recharged to aquifers in water-short areas.

3.1.3 Groundwater Quality

The value of the groundwater resource can be completely or partially lost whengroundwater is contaminated. While a spill of chemicals to a river can cause greatdamage, the immediate threat is short lived: the flowing stream can flush out thechemical in a matter of days. In contrast, we have seen that groundwater movesslowly. Thus, a spill to a groundwater aquifer usually persists for many years asthe chemical slowly spreads in the aquifer. Further, water can be rendered unfit fordrinking by only small amounts of chemicals. For example, benzene, a componentof gasoline, is limited to a concentration of 5 μg/L in drinking water by U.S. Envi-ronmental Protection Agency regulations. This is equivalent to 5 parts of benzene ina billion parts of water—about the same as 1 drop of benzene in a railroad tank carof water. A very small spill can thus contaminate a very large quantity of water.

Water quality can be compromised by more than just an isolated spill. Systemicpractices may also cause problems. Some examples that have affected cities arewidespread discharge of wastewater to subsurface disposal systems and ground-water overdraft in coastal cities leading to salt-water intrusion. Areas transitioningfrom agricultural to urban land use may have legacies of excessive pesticides ornitrogen-based fertilizers in the groundwater, and salt accumulation from past irri-gation practices. Finally, it is important to recognize that some groundwater is nat-urally contaminated by high mineral concentrations.

Compromises to groundwater quality can often be remedied by treatment.Indeed, groundwater used for municipal drinking water is always treated by dis-infection and additional treatment is common. Other uses of groundwater mayor may not require treatment—agricultural use is not nearly as demanding as

36 P. Shanahan

drinking-water use while some specialized industrial uses require more extensivetreatment than drinking water. Whatever the use, treatment should be recognized asa viable option to enabling use of groundwater.

3.1.4 Additional Reading

There are many good references that provide additional information on ground-water. A very accessible and comprehensive reader is a U.S. Geological Surveyreport by Heath (1982), available on-line. Advanced texts on groundwater hydrol-ogy and hydrogeology include Freeze and Cherry (1979), Todd and Mays (2005),Fetter (2001), and Domenico and Schwartz (1998). A very practical field-orientedreference is Driscoll (1986) and an excellent and very accessible text on contami-nants in groundwater is Fetter (1999).

3.2 Groundwater in Cities

Up to this point in the chapter, we have been discussing groundwater only in generaland not focusing on its place in the urban environment. That place is subtle and usu-ally hidden, but can be significant to the overall life of the city. Natural hydrologicprocesses are greatly modified in the urban environment, with consequent effects ongroundwater. The city disrupts the natural cycle in which precipitation recharges theaquifer and groundwater flows through the aquifer from recharge areas to dischargeareas along streams and rivers. In the city, roads, buildings, and other imperviousstructures cover the land surface and reduce recharge. But, the city’s water infras-tructure (water-supply and sewer pipes) may leak water in some locales, providinglocalized sources of recharge (some potentially contaminated). Irrigated parks andgardens may also act as pockets of concentrated recharge. On the discharge side ofthe cycle, withdrawals from wells that supply the city may capture the groundwaterthat would otherwise flow to streams and rivers. Subsurface utilities and structuresmay act as drains that draw water from the ground. These various disruptions ofthe recharge and discharge components of the natural system must all be accountedfor when considering the interaction between groundwater and the city. Figure 3.3illustrates these interactions in a schematic chart of flows to and from the urbanaquifer. Still another potential disruption is to the character of the subsurface. Filledland may include bricks and other debris, with large openings and much higherhydraulic conductivity than the natural soils, thus creating artificial pathways forpreferential flow of groundwater.

3.2.1 Phases in the Relationship Between a City and ItsGroundwater

Before considering how urban infrastructure interacts with the groundwater system,we must recognize that the city is not static—it grows and changes with time, and


Flow to surface water

Flow to adjacent aquifers

Indirect Recharge

InfiltrationIndirect

RechargeArtificial

Recharge

Land Drainage

Leakage Infiltration

Precipitation

Surface Water Bodies

Adjacent Aquifers

Dry wells Sewers Mains

Domestic / industrial

consumers Irrigation

Ground-water withdrawals

EvaporationEvaporation

Change of Storage

Inputs

Outputs

Natural Processes Artificial Processes

Aquifer

Fig. 3.3 The urban aquifer water balance (Shanahan and Jacobs, 2007, adapted from Simpson,1994)

its effects upon the groundwater system change with time. This section considersthe different phases in how a city interacts with the underlying groundwater, usingLondon, England as an example.

The pre-industrial city—using local groundwater: The nascent city is small anddecentralized. Even after people have started to gather in a single locale, the ser-vices and infrastructure that we typically associate with mature cities take time todevelop. Thus, residents in the young city typically draw upon springs, streams, andlocal shallow aquifers to self-supply water. Simultaneously, they develop cesspoolsand septic systems to dispose of wastewater on a residence-by-residence basis. Mostof today’s great cities had such humble beginnings. London, for example, was estab-lished by the Romans around 50 AD, deriving its water from the Thames River andseveral small streams and springs within the town. This local supply sufficed forabout twelve hundred years (Richards and Payne, 1899).

While local water supply and local wastewater disposal typify the first phase ofurban development, these methods are unsustainable for growing cities and invari-ably fail to keep up with population growth. In London by the early thirteenthcentury, the original supplies were no longer adequate and conduits began to be con-structed to carry water from outlying districts to public cisterns in the city proper.Further improvements commenced in 1582 when Peter Morrys constructed beneathLondon Bridge a water wheel that pumped Thames River water into a small networkof pipes servicing a part of the city. Other systems followed, including in the first twodecades of the 1600s construction of the New River, a 68-kilometer-long artificialcanal to bring water to the city from groundwater springs to the north (Ward, 2003).Despite these beginnings of London’s modern water-supply infrastructure, somecity residents still depended upon wells within the city through most of the 1800s.

The experience of London in the Victorian era—around the mid-1800s—pointsout another force that moved cities away from local groundwater supplies: ground-water pollution. In 1854, Dr. John Snow showed the link between a cholera epidemic

38 P. Shanahan

in the Golden Square neighborhood of London and consumption of water from theBroad Street well, an 8.5 m-deep well tapping the shallow aquifer. The Broad Streetwell was immediately adjacent to and in fact hydraulically connected to a cesspool,which proved to be the source of the bacterium that caused the cholera epidemic(Prescott and Horwood, 1935; Johnson, 2006). Snow’s findings eventually led tothe germ theory of disease and modern approaches to public health, including pro-tection of water supplies from pollution. Groundwater pollution can be extensivewhere on-site disposal is the predominant means of handling domestic wastewa-ter as well as near industrial facilities. Lawrence et al. (2000) cite a modern-dayexample in the developing world: wastewater discharges from the city of Hat Yai,Thailand have so altered the quality and chemistry of the underlying aquifer as todissolve natural arsenic and further contaminate the aquifer.

The industrial city—falling groundwater levels: The initial phase of urban effectson groundwater thus gives way to a second phase in which local supplies are inad-equate or too contaminated to provide water from within the city. Often, this phasecoincides with industrial development and the increase in water demands that indus-try brings. The second phase of urban effect is thus one in which the water infras-tructure is more or less fully developed. A water distribution system is in placealong with wastewater collection. Both depend upon high-volume centralized facil-ities for water supply and wastewater treatment. In London, the second half of thenineteenth century led to the development of a modern infrastructure for watersupply and wastewater collection and disposal. Water supply was developed firstby private companies that initially competed fiercely for business, later achieved atruce by which each company serviced a particular area within London, and finallywere consolidated into a single public entity (Ward, 2003). By 1866, under the far-sighted leadership of engineer Joseph Bazalgette, the city constructed a compre-hensive sewer system for collection and disposal of the city’s wastewater (Halliday,1999). Storm sewers also replaced natural streams; the Fleet River that once flowedthrough the center of the old city of London was now hidden, buried in an under-ground brick tunnel as much as 8 m below the surface (Barton, 1992, p. 26).

For the urban aquifer, the second phase of urban development is often a periodduring which the water table is lowered as regional (usually deep) groundwateraquifers are tapped. The regional Chalk aquifer underlies southwest England includ-ing London. It is about 180 to 240 m thick and is in most places confined by the60 m-thick London Clay formation (Walters, 1936). While some wells drew fromthe Chalk aquifer as early as 1800 (Clow, 1989), beginning in 1846, a numberof deep wells were constructed to tap the aquifer north of London (Ward, 2003,p. 181). Walters (1936) reports that by the mid-1930s there were 750 wells in theLondon metropolitan area drawing over ten thousand m3/day. This large and exten-sive withdrawal resulted in a general lowering of the water level in the aquifer by 15m or more through most of the metropolitan area and over 45 m at a pumping centernorth of the city (Fig. 3.4).

The falling groundwater levels beneath the industrial city have many effects. Oneparticular problem is land subsidence. In London, the layer of London Clay hasshrunk as it has become dewatered, leading to a lowering of the land surface in


Fig. 3.4 Decline in water level (in feet; 1 ft = 0.3 m) in the London Basin Chalk aquifer (Walters,1936)

Central London by 20 to 25 cm since 1865 (Clow, 1989) with localized maximumsettlement of about 50 cm (Downing, 1994). Other cities have seen much moredramatic changes—Mexico City has subsided by meters—but even a fraction of ameter is enough to create potential structural problems for buildings that experienceuneven settling. There are other problems as well: supply wells must be deepenedas water levels fall; pumping costs increase because groundwater must be liftedmore; salt water may intrude into previously fresh aquifers in coastal areas; andthe bearing strength of foundation soils may change as soils become unsaturated(Downing, 1994). A peculiarly insidious problem is the dewatering of the woodpiles that support structures in some areas of older cites (Lambrechts, 2000). Aslong as wood piles remain saturated by groundwater, they resist rot and can lastcenturies. If the water table is lowered below the pile tops, however, wood rot israpid and building support may collapse in a matter of years.

Overall, the lowering of water tables implies a loss in water-supply resilience:local supplies are no longer adequate or perhaps even available to supply the popu-lace during times of drought or water-supply emergency. The city becomes depen-dant, even hostage, to distant sources and the infrastructure needed to bring that

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faraway water to the city. Chapter 12 discusses this type of situation in Arizona inthe arid southwestern United States.

The post-industrial city—rising groundwater levels: The mature city often passesthrough a third phase in its relationship with its underlying groundwater. This phasecan be triggered by one or more factors including deliberate restoration of pastgroundwater levels, the transition to a post-industrial economy, the loss of ground-water supplies due to industrial contamination or overdraft, or the replacement ofnearby groundwater supplies with more distant safer supplies. The result is reduceduse of local groundwater and a slow, steady rise in groundwater elevation. Often thisis a desired outcome, but it can lead to problems. Such is the case in London wherea rising water table is now intruding on subsurface structures built during the timeof lower water table. Figure 3.5 shows the water level measured in an observationwell at Trafalgar Square in central London. Water levels reached their lowest levelsin the 1950s and early 1960s, but have since recovered nearly 50 m due to reducedgroundwater pumping.

As with falling groundwater levels, rising groundwater levels bring a host ofproblems (Johnson, 1994; Brassington, 1990). Rising water levels affect the struc-tural properties of soils and can reduce bearing capacity, cause swelling, and cre-ate hydrostatic uplift pressures. As it approaches the land surface, a rising waterlevel can intrude on structures, cause surface ponding, jeopardize on-site wastewater

1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Year

–90

–80

–70

–60

–50

–40

–30

Obs

erve

d w

ater

leve

l (m

eter

s be

low

mea

n se

a le

vel)

Fig. 3.5 Groundwater levels in observation well at Trafalgar Square, London, in meters belowmean sea level (data provided by DEFRA, 2007)


systems, infiltrate into sewers, and attack masonry and concrete. In karstic areaswhere soluble limestone formations are near the land surface, rising water levels candissolve the rock and create sinkholes. Most problematic of these are the interfer-ences with subsurface infrastructure, to the point that in London the major orga-nizations responsible for infrastructure have banded together to form the Gen-eral Aquifer Research, Development and Investigation Team (GARDIT) to managegroundwater levels beneath the central city (Cherry, 2007). Since the year 2000,pumping in London has been increased in order to bring water levels back downand prevent groundwater intrusion into subsurface structures.

The future city under climate change and population growth: Two trends willhave great influence on groundwater in the city of the future: climate changeand population growth in the world’s largest cities. Climate change will influencegroundwater primarily by changing the quantity and distribution of groundwaterrecharge. IPCC (2007) indicates that changes in climate include more frequentintense rainfall, more frequent drought, less rainfall at subtropical latitudes, andmore rainfall at tropical and high latitudes. The geographical variation of thesetrends implies different responses in different areas, a phenomenon addressed bynumerous region-specific studies in the technical literature (see Hiscock, 2005,pp. 302–316, for a summary of some of these studies). As the result of climatechange, many regions will experience less groundwater recharge. Even areas thatsee more rainfall overall may have less recharge if there is less frequent, moreintense rain that runs off the land surface rather than soaks into the soil. Climatechange may also increase salinity intrusion in coastal aquifers. One cause of thisis a rise in sea level (IPCC, 2007). An even greater influence would be reductionin recharge: lighter freshwater floats on heavier salt water in coastal areas and adrop in the surface elevation of the freshwater (as might be caused by a reductionin recharge) would have a roughly forty-fold greater effect than an equivalent risein the salt water level. Overall, disruption of groundwater recharge patterns has thepotential to reduce the availability of the groundwater resource in both coastal andinland areas.

Another phenomenon of great significance to cities in the future is the growthof cities generally and the megacities of 10 million people or more particularly.Cities of this size only appeared late in the twentieth century but there were 16new megacities by the year 2000 (WWAP, 2003). Most of these megacities lie inwater-short areas, often in developing countries. Moreover, they reflect a worldwidetrend of increasing population in cities of all sizes—54% of the population in theyear 2015 is forecasted to be urban versus only 38% in the year 1975. Consideringalso the overall increase in population (7.2 billion in 2015 from 4.1 billion in 1975),the urban population will have more than doubled by 2015 (from 1.5 billion to 3.9billion). Clearly, the world’s cities will need to provide much more water and dealwith much more wastewater and pollution potential as these trends continue. Withinthis context, groundwater supplies will be vulnerable to both greater pollution andoverexploitation.

Summary: We can define three phases in the relationship between a maturingcity and its underlying groundwater (as well as foresee a possible fourth phase

42 P. Shanahan

associated with climate change). The young city draws upon shallow groundwateras both its source of drinking water and its disposal medium for wastewater. Withpopulation growth and industrialization, this arrangement becomes unsustainableand the city moves to a second phase, drawing groundwater in quantity from distantor deep aquifers, often lowering groundwater levels dramatically. Finally, ground-water withdrawals decrease dramatically in the post-industrial city, causing ground-water levels to start rising, affecting subsurface infrastructure created in times oflower water tables.

3.2.2 Cities and Groundwater Quality

A large number of studies have shown that urban and suburban development isassociated with a degradation of groundwater quality (Shanahan and Jacobs, 2007).A variety of potential contamination sources exist in urban areas including chemicalhandling at industrial and commercial facilities, storage of petroleum fuels, on-sitedisposal of wastewater, infiltration of contaminated runoff, and application of deic-ing chemicals to roadways. Thus, there is in general a concentration of potentialsources in urban areas that contribute to overall higher pollutant loads. The charac-ter of the resulting problems is not unique to urban settings and their investigationand remediation is addressed in textbooks such as Fetter (1999) and Sharma andReddy (2004).

A water-quality issue unique to cities is the potential for leaking sewers tocontaminate the groundwater. Sanitary sewers are known to leak and the releasedsewage would clearly carry contaminants. There is scant evidence that leakingmunicipal sewers have caused actual observed instances of groundwater pollution(Lerner et al., 1994). Barrett et al. (1999) sampled for specific chemicals associatedwith sewage in an urban setting and failed to find a strong indication of leakage. Anexception from these general findings is the author’s experience in the case of indus-trial facilities that historically discharged toxic organic chemicals—leaking sewersat industrial facilities have been found to be sources of groundwater contaminationin investigations of contaminated sites. Overall, leaking sewers may be sources ofgroundwater contamination in some instances, but seem not to be a problematicgeneral source based on the limited information available.

3.2.3 Analysis of Urban Groundwater

Review of the previously cited textbooks on groundwater hydrology and hydroge-ology will show that analysis of groundwater is a well established discipline withcarefully developed tools and methods for analysis of groundwater systems. Forexample, there are known equations for predicting how the water table will drawdown (that is, become lower) after a pumping well starts to withdraw water froman aquifer. Unfortunately, these techniques require special care and attention whenapplied to the particular circumstances of the city.


The most unique aspect of urban hydrogeology is the degree to which the sub-surface has been modified by man. Most cities have been excavated and filled, arecrisscrossed by pipes and conduits of various kinds, may be pockmarked by oldunused water-supply wells, and are drained at tunnels and basements—in short,have been extensively modified in ways that affect the groundwater system. Con-sider construction of a sanitary sewer. First, a trench is cut into the earth. Next,a gravel bedding layer is placed in the bottom of the trench. Often, a drain lineis then installed within the gravel (and beneath the eventual sewer line) to drainaway groundwater during construction. Next, the sewer line itself is laid. At last,the trench is backfilled to restore the original land-surface grade. Throughout thisprocess, materials of high hydraulic conductivity have been placed in contiguousdeposits creating preferential pathways for flow of groundwater—mini-aquifers inessence. These mini-aquifers typically affect a local area only, but sometimes withdramatic effect. In Boston, Massachusetts, a sewer line along St. James Avenue low-ered the water table beneath the nearby public library and Trinity Church—two clas-sic buildings supported on wood piles. The lowered water table (Fig. 3.6) exposedthe tops of the wood piles, caused them to rot and jeopardized the underpinnings ofboth buildings (Snow, 1936).

The existence of preferential pathways along pipelines, conduits, and old wellsalso has implications for contaminant transport in groundwater. It is not uncom-

Fig. 3.6 Water-table contours (in ft; 1 ft = 0.3 m) along St. James Street, Boston in 1932 showingimpact of St. James Avenue sewer (Aldrich and Lambrechts, 1986)

44 P. Shanahan

mon for underground gasoline storage tanks to leak, releasing gasoline to the sub-surface. In an urban setting, the leaked product can follow a zigzagging pathwayfrom one street to the next as it travels along sewer trenches (Lundy and Gogel,1988). In an undisturbed geologic setting, we would expect entirely different behav-ior, with a tongue-shaped plume heading off in a single direction aligned with theregional groundwater flow. In contrast, the disparate fill material used in many citiescan create preferential pathways for groundwater flow and contaminant migration.Similarly, old wells can create preferential pathways for vertical flow, sometimesspreading contaminants from a contaminated shallow aquifer to deep uncontami-nated aquifers.

These examples illustrate a special challenge of urban groundwater analysis: sub-surface infrastructure can have significant but local effect on groundwater flow andcontaminant transport and must be taken into account. Unfortunately, the tools foranalyzing groundwater systems—computer models—typically look at larger spatialscales and are cumbersome for the street-by-street analysis needed for many urbangroundwater problems. For many problems, the urban groundwater analyst mustinvestigate and consider finer spatial details in order to understand the groundwatersystem. Thus there is an intrinsic knowledge gap in urban groundwater analysis:rarely is there sufficient detailed information to understand completely the urbangroundwater system.

The urban hydrologist must also account for the idiosyncrasies of particular cli-mates and locales. For example, several researchers have reported on the paradoxof groundwater flooding in desert cities (Shanahan and Jacobs, 2007). Over time,low-hydraulic-conductivity hard pans develop in desert soils from the residuals leftby evaporation. When a city is created atop these hard pans, the water added byleakage from water and sewer lines and irrigation of parks and gardens can pondon the hard-pan layer and cause surface flooding and water intrusion into buildings.Kuwait City, for example, has seen water tables rise as much as 5 m (Al-Rashed andSherif, 2001). Local conditions have caused unique responses to altered ground-water hydrology in other locations as well. Harking back to prior examples in thischapter, the dramatic subsidence in Mexico City stems from its location over thickclay beds while the contamination by arsenic in Hat Yai, Thailand is a result ofits particular hydrogeology and geochemistry. Thus, while general principles apply,site-specific conditions must always be considered.

3.2.4 Managing Urban Groundwater

Urban groundwater has been, and for the most part remains, unmanaged and unreg-ulated (Shanahan and Jacobs, 2007). The United Kingdom is typical: extraction ofgroundwater was considered a right under common law (Walters, 1936) and wasessentially unregulated until passage of the National Water Resources Act of 1963and initiation of a system to license water abstraction (EA, 2002). With no centralcontrol prior to 1963, groundwater was overexploited and the groundwater resourcediminished as water levels fell. In the 1960s, controls on groundwater abstraction


along with reduced industrial activity halted the decline in water levels and, eventu-ally, led to rising water levels.

A lack of rules for preventing overexploitation or contamination of groundwater,or a failure to enforce the rules, is the norm, particularly during periods of rapid eco-nomic development. Groundwater is perhaps the ultimate “out of sight, out of mind”resource. It is difficult and expensive to monitor and manage, and there is usuallyno oversight until some crisis intervenes. In London, it was the extreme fall in waterlevels that led to action in the 1960s to prevent overexploitation. Today, the issueis rising water levels and only informal controls are in place to manage it. In 1992,London Underground, the National Rivers Authority and Thames Water formed theGARDIT to address the rising groundwater that threatened subsurface infrastruc-ture (Stedman, 1999). The Environment Agency, in turn, still operating under the1963 Water Management Act, instituted in 2002 a comprehensive system for man-aging water abstraction known as the Catchment Abstraction Management Strategy(CAMS) (EA, 2002, 2006). GARDIT now cooperates with the Environment Agencywithin the CAMS framework to pump groundwater from selected wells in Londonso as to manage groundwater levels (Cherry, 2007). The result is a comprehensivelybut informally managed aquifer.

London’s comprehensive management structure is the exception and most otherjurisdictions around the world still leave groundwater unmanaged. For the most part,regulatory systems are absent, but even when present, are often unenforced. In astudy of a groundwater basin in West Java, Indonesia, Braadbaart and Braadbaart(1997) blame groundwater overexploitation on a lack of enforcement rather than alack of regulatory structure. Thus, in addition to the technical challenges discussedabove, managing urban groundwater also presents a special political challenge.

How can we improve the management of ground water and increase the resilienceof the groundwater component of the urban water system? Clearly, prior systemmanagement has not been resilient: groundwater has boomeranged through differ-ent problems at different times, even changing from an important resource in pre-industrial cities to a nuisance in post-industrial cities. In large measure, groundwateris a problem because it is “out of sight and out of mind.” The public and even deci-sion makers know little about the state of the groundwater resource. Thus, adversechanges in groundwater quality or elevation can fester for years or decades withoutaction or even notice.

Technology offers one potential remedy. While several cities with criticalgroundwater issues, including London and Boston, monitor groundwater elevationson a regular basis, we have identified no instances in which cities provide web-baseddepiction of spatial groundwater conditions. The U.S. Geological Survey (USGS,2008) offers real-time data from individual monitoring wells—something poten-tially very useful for evaluating trends—but does not provide maps on the scaleof an individual city. Such information could remedy the out-of-sight, out-of-mindproblem and allow for more resilient management of the resource. For example,homeowners in landfilled sections of Boston worry about construction dewateringas a potential cause of compromise to the integrity of the wood piles that supporttheir homes. Easy access and visualization of groundwater data could allow them

46 P. Shanahan

(and construction managers) to evaluate impacts and make adjustments in real time.In water-short areas such tools could permit adaptive management of the conjunc-tive use of ground and surface water.

The out-of-sight, out-of-mind issue is also raised by Drangert and Cronin (2004)as partly responsible for past pollution of groundwater resources. They advocate anew management paradigm that focuses on households as a source of groundwaterpollutants and involves residents in preventing toxic chemicals, detergents, oil, andpharmaceuticals from being discharged with wastewater. Here as well, more acces-sible information could play an important part in educating the public and therebysecuring a more resilient groundwater supply. Indeed, better groundwater manage-ment and control of pollution can create a new resource. As pointed out by Fosterand Chilton (2004), urban wastewater is probably the only water resource that isbecoming more available. Thus, intelligent management of water reuse and artifi-cial groundwater recharge will inevitably become an increasingly important part ofurban water management.

While it is expensive and time-consuming to develop information systems thatcould enable better management of groundwater resources, pollution prevention,and conjunctive use, such tools offer great potential for adaptive management of theurban water system, including its little noticed groundwater component. The needfor such systems can only grow as the dual forces of climate change and growingurban population increase pressure on the groundwater resources of cities.

References

Aldrich, H. P., and J. R. Lambrechts. 1986. Back Bay Boston, Part II: Groundwater levels. CivilEngineering Practice 1:31–64.

Al-Rashed, M.F., and M.M. Sherif, 2001. Hydrogeological aspects of groundwater drainage of theurban areas in Kuwait City. Hydrological Processes 15: 777–795.

Barrett, M. H., K. M. Hiscock, S. Pedley, D. N. Lerner, J. H. Tellam, and M. J. French. 1999.Marker species for identifying urban groundwater recharge sources: a review and case study inNottingham, UK. Water Research 33:3083–3097.

Barton, N. 1992. The Lost Rivers of London, Second Edition. Historical Publications Ltd., London.Brassington, F. C. 1990. Rising groundwater levels in the United Kingdom. Proceedings of the

Institution of Civil Engineers, Part 1 88:1037–1057.Cherry, H. 2007. Groundwater Levels in the Chalk-Basal Sands Aquifer of the London

Basin. Environment Agency, Bristol. http://www.environment-agency.gov.uk/ common-data/acrobat/gw report june2007 1831580.pdf. Accessed September 18, 2007.

Clow, D. G. 1989. Rising groundwater in urban areas—implications for British Telecom. BritishTelecommunications Engineering 8:176–181.

DEFRA, 2007. e-Digest Statistics about: Inland Water Quality and Use, Water levels.Department for Environment, Food and Rural Affairs, London. http://www.defra.gov.uk/environment/statistics/inlwater/iwlevels.htm. Accessed September 28, 2007.

Domenico, P. A., and F. W. Schwartz, 1998. Physical and Chemical Hydrogeology, Second Edition.John Wiley & Sons, New York.

Downing, R. A. 1994. Keynote paper: Falling groundwater levels—a cost-benefit analysis,pp. 213–236. In: W. B. Wilkinson, editor. Groundwater Problems in Urban Areas. Proceedingsof the International Conference organised by the Institution of Civil Engineers and held inLondon, 2–3 June 1993. Thomas Telford, London.


Drangert, J. O., and A. A. Cronin, 2004. Use and abuse of the urban groundwater resource: Impli-cations for a new management strategy. Hydrogeology Journal 12: 94–102.

Driscoll, F. G., 1986. Groundwater and Wells, Second Edition. Johnson Division, St. Paul,Minnesota.

EA, 2002. Managing Water Abstraction: The Catchment Abstraction Management Strategy pro-cess. Environment Agency, Bristol, United Kingdom. July 2002. (http://www.environmentagency.net/commondata/acrobat/mwa˙english.pdf)

EA, 2006. The London Catchment Abstraction Management Strategy Final Strategy Document.Environment Agency, Bristol, United Kingdom. April 2006. (http://publications.environment-agency.gov.uk/pdf/GETH1205BJYD-e-e.pdf)

Fetter, C. W., 1999. Contaminant Hydrogeology, Second Edition. Prentice Hall, Upper SaddleRiver, New Jersey.

Fetter, C.W. 2001. Applied Hydrogeology, Fourth Edition. Prentice Hall, Upper Saddle River, NewJersey.

Foster, S. S. D., and P. J. Chilton, 2004. Downstream of downtown: urban wastewater as ground-water recharge. Hydrogeology Journal 12:115–120.

Freeze, R. A., and J. A. Cherry, 1979. Groundwater. Prentice Hall, Englewood Cliffs, New Jersey.Halliday, S., 1999. The Great Stink of London: Sir Joseph Bazalgette and the Cleansing of the

Victorian Capital. Sutton Publishing, Gloucestershire, UK.Heath, R. C., 1982. Basic Ground-Water Hydrology. Water-Supply Paper 2220. U.S. Geological

Survey, Washington, D.C. (http://water.usgs.gov/pubs/wsp/wsp2220/)Hiscock, K. 2005. Hydrogeology: Principles and Practice. Blackwell Publishing, Malden, Mas-

sachusetts.Hutson, S. S., N. L. Barber, J. F. Kenny, K. S. Linsey, D. S. Lumia, and M. A. Maupin. 2004.

Estimated Use of Water in the United States in 2000. Circular 1268. U.S. Geological Survey,Reston, Virginia. (http://pubs.usgs.gov/circ/2004/circ1268/)

IPCC, 2007. Climate Change 2007. The Physical Science Basis. Contribution of Working Group Ito the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cam-bridge University Press, Cambridge, United Kingdom. (http://ipcc-wg1.ucar.edu/wg1/wg1-report.html)

Johnson, S., 2006. The Ghost Map: The Story of London′s Most Terrifying Epidemic—and Howit Changed Science, Cities, and the Modern World. Riverhead Books, New York.

Johnson, S. T. 1994. Keynote paper: Rising groundwater levels: engineering and environmentalimplications, pp. 285–298. In: W. B. Wilkinson, editor. Groundwater Problems in Urban Areas.Proceedings of the International Conference organised by the Institution of Civil Engineers andheld in London, 2–3 June 1993. Thomas Telford, London.

Lambrechts, J. R. 2000. Investigating the Cause of Rotted Wood Piles, pp. 590–599. In: K. L.Rens, O. Rendon-Herrero, and P. A. Bosela, editors. Forensic Engineering, Proceedings ofthe Second Conference, San Juan, Puerto Rico. American Society of Civil Engineers, Reston,Virginia.

Lawrence, A. R., D. C. Gooddy, P. Kanatharana, W. Meesilp, and V. Ramnarong. 2000. Ground-water evolution beneath Hat Yai, a rapidly developing city in Thailand. Hydrogeology Journal8:564–575.

Lerner, D.N., D. Halliday, and M. Hoffman, 1994. The impact of sewers on groundwater quality,pp. 64–75. In: W. B. Wilkinson, editor. Groundwater Problems in Urban Areas. Proceedingsof the International Conference organised by the Institution of Civil Engineers and held inLondon, 2–3 June 1993. Thomas Telford, London.

Lundy, D., and T. Gogel. 1988. Capabilities and limitations of wells for detecting and monitor-ing liquid phase hydrocarbons, pp. 343–362. In: Proceedings of the Second National OutdoorAction Conference on Aquifer Restoration, Ground Water Monitoring and Geophysical Meth-ods. National Water Well Association, Dublin, Ohio, Las Vegas, Nevada.

48 P. Shanahan

Mercer, J. W., S. D. Thomas, and B. Ross, 1982. Parameters and Variables Appearing in RepositorySiting Models. NUREG/CR-3066. U.S. Nuclear Regulatory Commission, Washington, D.C.December 1982.

Prescott, S. C., and M. P. Horwood. 1935. Sedgewick′s Principles of Sanitary Science and PublicHealth. The Macmillan Company, New York.

Richards, H. C., and W. H. C. Payne. 1899. London Water Supply being a Compendium of the His-tory, Law, & Transactions relating to the Metropolitan Water Companies from Earliest Timesto the Present Day, Second Edition. P.S. King & Son, London.

Shanahan, P., and B. L. Jacobs. 2007. Ground water and cities. In: V. Novotny and P. Brown, edi-tors. Cities of the Future: Towards Integrated Sustainable Water and Landscape Management.IWA Publishing, London.

Sharma, H. D., and K. R. Reddy. 2004. Geoenvironmental Engineering. John Wiley & Sons,Hoboken, New Jersey.

Simpson, R. W. 1994. Keynote paper: Quantification of processes, pp. 105–120. In: W. B. Wilkin-son, editor. Groundwater Problems in Urban Areas. Proceedings of the International Con-ference organised by the Institution of Civil Engineers and held in London, 2–3 June 1993.Thomas Telford, London.

Snow, B. F. 1936. Tracing loss of groundwater. Engineering News Record 117:1–6.Stedman, L., 1999. Businesses act in attempt to keep London dry. Water & Waste Treatment 40(5):

16 (http://www.edie.net/library/view˙article.asp?id=1845).Todd, D. K., and L. W. Mays, 2005. Groundwater Hydrology, Third Edition. John Wiley & Sons,

Hoboken, New Jersey.USGS, 2008. USGS Ground-Water Data for the Nation (http://waterdata.usgs.gov/nwis/gw.

Accessed June 9, 2008).Ward, R. 2003. London′s New River. Historical Publications Ltd., London.Walters, R. C. S., 1936. The Nation’s Water Supply. Ivor Nicholson and Watson Limited, London.Winter, T. C., J. W. Harvey, O. L. Franke, and W. M. Alley, 1998. Ground Water and Sur-

face Water: A Single Resource. Circular 1139. U.S. Geological Survey, Denver, Colorado(http://water.usgs.gov/pubs/circ/circ1139/).

WWAP, 2003. Water for People, Water for Life, The United Nations World Water DevelopmentReport. World Water Assessment Programme, UNESCO.

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